Polymer membranes have been proposed for various separations. It has been found that different molecules can be made to diffuse through selected polymers differently. For example if one component of a mixture is found to diffuse through a polymer rapidly and a second component is found to diffuse through the polymer very slowly or not at all, the polymer may be utilized to separate the two components. Polymer membranes potentially can be used for gas separations as well as liquid separations.
Polymeric membrane materials have been found to be of use in gas separations. Numerous research articles and patents describe polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyethers, polyamides, polyarylates, polypyrrolones, etc.) with desirable gas separation properties, particularly for use in oxygen/nitrogen separation (See, for example, Koros et al., J. Membrane Sci., 83, 1-80 (1993), the contents of which are hereby incorporated by reference, for background and review).
The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater mole fraction of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
The relative ability of a membrane to achieve the desired separation is referred to as the separation factor or selectivity for the given mixture. There are however several other obstacles to use of a particular polymer to achieve a particular separation under any sort of large scale or commercial conditions. One such obstacle is permeation rate. One of the components to be separated must have a sufficiently high permeation rate at the preferred conditions or else extraordinarily large membrane surface areas are required to allow separation of large amounts of material. Another problem that can occur is that at conditions where the permeability is sufficient, such as at elevated temperatures or pressures, the selectivity for the desired separation can be lost or reduced. Another problem that often occurs is that over time the permeation rate and/or selectivity is reduced to unacceptable levels. This can occur for several reasons. One reason is that impurities present in the mixture can over time clog the pores, if present, or interstitial spaces in the polymer. Another problem that can occur is that one or more components of the mixture can alter the form or structure of the polymer membrane over time thus changing its permeability and/or selectivity. One specific way this can happen is if one or more components of the mixture causes plasticization of the polymer membrane. Plasticization occurs when one or more of the components of the mixture acts as a solvent in the polymer often causing it to swell and lose its membrane properties. It has been found that polymers such as polyimides which have particularly good separation factors for separation of mixtures comprising carbon dioxide and methane are prone to plasticization over time thus resulting in decreasing performance of the membranes made from the polyimides.
The present invention overcomes some of the problems of the prior art membranes by providing a polymer membrane and a route to making said polymer membrane that has the following properties/advantages:
a) Excellent selectivity and permeability,
b) Sustained selectivity over time by resistance to plasticization, and
c) Very large useable surface area by use of hollow fibers.